1. Field of the Invention
The present invention relates to a nickel-free white copper alloy having excellent strength, hardness, ductility, workability and corrosion resistance, suitable for use in elements, sliders, stoppers or the like for slide fasteners, or accessories such as metallic buttons, fasteners or the like for clothes, causing no allergic problem and having high whiteness.
2. Description of the Prior Art
As conventional copper alloys, for example, for the above-mentioned fasteners, copper-nickel-zinc alloys such as nickel silver, which has a white alloy hue, or copper-zinc alloy represented by red brass or brass have been used. Since nickel silver contains nickel as an alloying element, corrosion resistance is excellent. However, for example, where this is applied to the use as a slide fastener, the fastener often contacts with a skin, and there arises the allergic problem due to nickel. Whereas the copper-zinc alloy represented by red brass or brass does not cause such an allergic problem since it contains no nickel. However, its color tone becomes yellowish, and a white alloy cannot be obtained.
Accordingly, an object of the present invention is to provide a white copper alloy having excellent strength and hardness equal to those of nickel silver, as well as excellent workability, corrosion resistance and whiteness in addition to ductility, and having no allergic problem because the alloy contains no nickel.
The present invention comprises the following (1)-(5).
(1) A nickel-free white copper alloy represented by the general formula: CuaZnbMncAld, wherein b, c, and d are 0.5xe2x89xa6b less than 5, 7xe2x89xa6cxe2x89xa617 and 0.5xe2x89xa6dxe2x89xa64 in terms of % by weight; and a is the balance, the alloy incidentally including unavoidable elements.
(2) A nickel-free white copper alloy, represented by the general formula: CuaZnbMncAldXe, wherein X is at least one element selected from the group consisting of Si, Ti and Cr; b, c, d and e are 0.5xe2x89xa6b less than 5, 7xe2x89xa6cxe2x89xa617, 0.5xe2x89xa6dxe2x89xa64 and 0 less than exe2x89xa60.3 in terms of % by weight; and a is the balance, the alloy incidentally including unavoidable elements.
(3) The nickel-free white copper alloy as described in the above (1) or (2), wherein the b, c and d are 0.5xe2x89xa6b4, 7xe2x89xa6cxe2x89xa615 and 0.5xe2x89xa6dxe2x89xa62 in terms of % by weight.
(4) The nickel-free white copper alloy as described in any one of the above (1), (2) and (3), wherein the alloy is a single xcex1-phase state at room temperature.
(5) The nickel-free white copper alloy as described in any one of the above (1), (2) and (3), wherein the alloy has such a color tone that a* value and b* value representing a color tone defined by JIS Z 8729 are 0 less than a* less than 5 and 7 less than b* less than 15.
In the composition of the present invention, Zn has an effect of improving the mechanical properties of the alloy through its solid solution strengthening effect and also a cost reduction effect of the alloy. If the Zn content is less than 0.5%, the cost reduction effect and the strengthening effect are insufficient. If the content is more than 5%, the solid- solution coexistence temperature range becomes broad and macro segregation tends to be marked. Also, heat conduction and castability tend to decrease. Further, when the Zn content is larger than 5%, season cracking resistance deteriorates and also the crystalline structure becomes an xcex1+xcex2 phase, so that a sufficient cold-workability cannot be secured. By setting to 5% or less, the problem of season cracking does not occur and a more stable state can be maintained even if the X element defined in the aforesaid general formula CuaZnbMncAldXe is added. 4% or less is more preferred. Mn has effects in providing improved mechanical properties to the alloy by the solid solution strengthening effect and also in cost reduction of the alloy. Further, by addition of Mn in the above-specified amount as a partial replacement of zinc, there occur the effect of improving the season cracking resistance as well as the effect of preventing the color tone of the copper alloy from turning to yellowish excessively. Further, it has an effect of lowering the melting point of the alloy, thus improving the castability and also suppressing vaporization of zinc from a melt. If it is less than 7%, the color tone becomes yellowish. Conversely, if it is larger than 17%, the crystal structure becomes an xcex1+xcex2 phase, so that a sufficient cold-workability cannot be secured. The upper limit of Mn is more preferably 15%.
Al has an effect of improving the season cracking resistance by forming a stable oxide film on the alloy surface. Further, it improves the mechanical properties of the alloy through the solid solution strengthening effect and also decreases the cost of the alloy. The lower limit of the Al amount is 0.5%. When the amount is too small, the season cracking resistance and the strengthening effect become insufficient. On the other hand, if it is larger than 4%, the crystalline structure becomes an xcex1+xcex2 phase, so that a sufficient cold-workability cannot be secured. 2% or less is more preferred.
The Element X (at least one element selected from the group consisting of Si, Ti and Cr) in the general formula CuaZnbMncAldXe serves to form a coating on a melt surface during melting, and also serves to prevent oxidation of Mn and vaporization of Zn. Further, by forming a stable oxide coating on the alloy surface, there occur the functions of preventing elimination of Mn during annealing and improving the season cracking resistance and also the effect of preventing change in color tone with the lapse of time due to oxidation of Mn. The lower limit of the amount of the element X is more than 0%. However, if the amount is too small, the above effects are not sufficiently obtained. Therefore, the amount is preferably 0.02% or more. If the amount is larger than 0.3%, an intermetallic compound is formed with elements in the composition, causing deterioration of cold-workability.
The present invention alloy is composed a single xcex1-phase, and can secure a sufficient cold-workability. If outside the composition range of the present invention, the crystalline structure tends to be an xcex1+xcex2 phase, and the workability lowers.
Further, the present invention alloy is in ranges of 0 less than a* less than 5 and 7 less than b* less than 15 based on the chromaticity diagram of the (L* a* b*) colorimetric system if defined by JIS Z 8729.
The color tone mentioned in the present specification is shown by the values of psychometric lightness index L* (lightness: L star) and psychometric chromaticity indexes a* (greenish-reddish: a star) and b* (bluish-yellowish: b star) expressed in accordance with the specification of color of materials defined by JIS Z 8729. In particular, in order to be white color that is the characteristic of the present invention, it is better to be a color near achromatic color, which can be defined by the chromaticity indexes a* and b* as mentioned above.
The present invention is explained specifically below based on the examples.